1. Field of the Invention
The present invention relates to the papermaking arts. More specifically, the present invention relates to a papermaker's fabric of the on-machine-seamable variety, such as an on-machine-seamable press fabric for the press section of a paper machine.
2. Description of the Prior Art
During the papermaking process, a fibrous web is formed by depositing a fibrous slurry, that is, an aqueous dispersion of cellulosic fibers, on a moving forming fabric in the forming section of a paper machine. A large amount of water is drained from the slurry through the forming fabric during this process, leaving the fibrous web on the surface of the forming fabric.
The newly formed web proceeds from the forming section to a press section, which includes a series of press nips. The fibrous web passes through the press nips supported by a press fabric, or, as is often the case, between two press fabrics. In the press nips, the fibrous web is subjected to compressive forces which squeeze water therefrom, and which adhere the fibers in the web to one another to turn the fibrous web into a sheet. The water is accepted by the press fabric or fabrics and, ideally, does not return to the web.
The web, now a sheet, finally proceeds to a dryer section, which includes at least one series of rotatable dryer drums or cylinders, which are internally heated by steam. The sheet itself is directed in a serpentine path sequentially around each in the series of drums by a dryer fabric, which holds the web closely against the surfaces of the drums. The heated drums reduce the water content of the sheet to a desirable level through evaporation.
It should be appreciated that the forming, press and dryer fabrics all take the form of endless loops on the paper machine and function in the manner of conveyors. It should further be appreciated that paper manufacture is a continuous process which proceeds at considerable speed. That is to say, the fibrous slurry is continuously deposited onto the forming fabric in the forming section, while a newly manufactured paper sheet is continuously wound onto rolls after it exits from the dryer section.
Referring, for the moment, specifically to press fabrics, it should be recalled that, at one time, press fabrics were supplied only in endless form. This is because a newly formed paper sheet is extremely susceptible to marking in the press nip by any nonuniformity in the press fabric or fabrics. An endless, seamless fabric, such as one produced by the process known as endless weaving, has a uniform structure in both its longitudinal (machine) and transverse (cross-machine) directions. A seam, such as a seam which may be used to close the press fabric into endless form during installation on a paper machine, represents a discontinuity in the uniform structure of the press fabric. The use of a seam, then, greatly increases the likelihood that the paper sheet will be marked in a press nip.
Accordingly, the seam region of any workable on-machine-seamable press fabric must generally behave under load, that is, under compression in the press nip or nips, like the rest of the press fabric, and must have the same permeability to water and to air as the rest of the press fabric, in order to minimize, if not entirely avoid, the marking of the paper product being manufactured by the seam region.
Despite the considerable technical obstacles presented by these requirements, it remained highly desirable to develop an on-machine-seamable press fabric, because of the comparative ease and safety with which it could be installed on the press section. Ultimately, these obstacles were overcome with the development of press fabrics having seams formed by providing seaming loops on the crosswise edges of the two ends of the fabric. The seaming loops themselves are formed by the machine-direction (MD) yarns of the fabric. A seam is formed by bringing the two ends of the press fabric together, by interdigitating the seaming loops at one end of the fabric with those at the other end, and by directing a so-called pin, or pintle, through the passage defined by the interdigitated seaming loops to lock the two ends of the fabric together. Needless to say, it is much easier and far less time-consuming to install an on-machine-seamable press fabric, than it is to install an endless press fabric, on a paper machine.
There are several methods for producing a press fabric that can be joined into endless form on the paper machine with such a seam. One method is to flat-weave the press fabric, in which case the warp yarns would be its machine-direction (MD) yarns. To form the seaming loops in this method, the warp ends are woven some distance back into the fabric body in a direction parallel to the warp yarns. Another technique, far more preferable, is a modified form of endless weaving, which normally is used to produce an endless loop of fabric. In modified endless weaving, the weft, or filling, yarns are continuously woven back and forth across the loop, in each passage forming a loop on one of the edges of the fabric being woven by passing around a loop-forming pin. As the weft yarn, or filling yarn, which ultimately becomes the MD yarn in the press fabric, is continuous, the seaming loops obtained in this manner are stronger, and the seam formed with such seaming loops has a greater load-bearing capacity, than any that can be produced by weaving the warp ends back into the ends of a flat-woven fabric. In still another method, a fabric is woven endless, and the endless loop of fabric thereby obtained is flattened and given the form of two fabric layers joined to one another at two widthwise ends of the flattened loop. One or more widthwise yarns are then removed from each of the two widthwise ends to produce a short gap defined by the freed, that is, the newly unwoven portions of, lengthwise yarns at each end. These unwoven portions of the lengthwise yarns are then used as seaming loops when the two widthwise ends are brought together as described above.
For the manufacture of certain paper grades, such as linerboard and corrugating media, press fabrics having high void volume and high water-handling capacity are needed. Multi-layered press fabrics, such as those shown in U.S. Pat. Nos. 4,865,083 and 4,887,648 to Cunnane, are useful for such applications.
These patents issued from related applications, and show a method for seam formation for a press fabric having at least four machine-direction layers. In the disclosed method, a woven base fabric, having at least four layers in the machine direction, is provided with seaming loops at each of two ends to be joined together. The seaming loops finish at least two of the layers. That is to say, at the two ends of the fabric, a machine-direction (MD) yarn from one of the at least four layers forms a seaming loop and reenters the fabric in another of the at least four layers, the same two layers being so related at each end, thereby finishing the two layers in question. The remaining layers are finished in a similar manner with or without seaming loops. By altering the choice of which of the layers are finished in seaming loops, the geometry of the seam can be varied.
The present invention is also a multi-layered press fabric, but is designed to be seamed into endless form during installation on a paper machine more readily than those shown in U.S. Pat. Nos. 4,865,083 and 4,887,648.